Short cervix outcomes?

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I'm 20 weeks/4 days with di/di boys. I found out yesterday at my anatomy ultrasound that my cervix is short at 20 mm. I have a repeat ultrasound next week and didn't get much information from the doctor I saw yesterday. Does anyone have a similar experience and can tell me what happens next or the experiences you had? I'm so nervous because I'm not even to viability yet.